1. Field of the Invention
The present invention relates to an electron beam system (such as an electron probe microanalyzer (EPMA)) or a scanning electron microscope (SEM) for directing a sharply focused electron beam at the surface of a specimen and gaining a backscattered electron image using electrons backscattered from the specimen to thereby permit the user to make an observation or analysis). More specifically, the invention relates to an electron beam system for gaining and displaying backscattered electron images having appropriate brightness and contrast and to a method of operating the system.
2. Description of Related Art
It is known that the intensity of electrons (known as backscattered electrons or reflected electrons) backscattered from a specimen irradiated with an electron beam is dependent on the average atomic number of the specimen. However, where the specimen surface is tilted, the direction in which the intensity of the backscattered electrons maximizes is substantially symmetrical with respect to the line normal to the specimen surface. Therefore, the intensity of backscattered electrons incident on a detector disposed only on one side of the tilted surface is greatly affected by the orientation of the tilted surface. Consequently, in practice, many instruments are equipped with backscattered electron imaging (BSEI) capabilities capable of separating information about the composition (average atomic number) from topographic information indicating the tilt of the surface.
FIGS. 9A and 9B illustrate the principle of a method of separating compositional information from the topographic information by this kind of backscattered electron imaging device. A backscattered electron detector assembly shown in FIG. 9A consists of a pair of detectors A and B having the same detection sensitivity. The detectors A and B are arranged symmetrically with respect to an electron beam directed at the specimen. As shown in FIG. 9B, if the sum (A+B) of the output signals from the detectors A and B is taken, information about the surface tilt is canceled out, while the compositional information is emphasized. Conversely, if the difference (A−B) is taken, the compositional information is canceled out, whereas the topographic information is emphasized. An image created from the sum signal (A+B) is known as a compositional BSE (backscattered electron) image or simply as a compositional image. An image created from the differential signal (A−B) is known as a topographic BSE image or simply as a topographic image. The relationship between the intensity of backscattered electron signal and the average atomic number is discussed on the assumption that the specimen is flat and smooth or that the sum output signal (A+B) shown in FIGS. 9A and 9B is utilized.
FIG. 10 is a graph of data obtained by measurements of signal intensities from a backscattered electron detector for the average atomic numbers of specimens under some different accelerating voltages. The horizontal axis indicates average atomic numbers. The vertical axis indicates the intensities of the backscattered electron signals. The signal intensities have been normalized based on a backscattered electron signal intensity obtained from copper having an average atomic number of 29. It can be seen that as the average atomic number is increased, the backscattered electron intensity increases monotonously. The dependence of the backscattered electron intensity on the atomic number is not linear in practice but, strictly speaking, has somewhat complex relationships because of a varying term of the minimum excitation voltage. However, within a limited range of atomic number differences, linear approximation is available in practical applications.
The dependence of backscattered electron intensity on atomic number is used in various kinds of analysis. For example, in Japanese Patent Laid-Open No. S52-117192, there is disclosed a technique of identifying the properties of carbons and cokes by making use of the relationships of backscattered electron intensity to hydrogen and carbon contents of coals and cokes. In Japanese Patent Laid-Open No. H8-201317, there is disclosed a technique of identifying compounds contained in a metal specimen by previously preparing a calibration curve from the relationship between a reference specimen having a known atomic number and the backscattered electron intensity and identifying the compounds from actually measured intensities of backscattered electrons. In Japanese Patent Laid-Open No. H8-148111, there is disclosed a technique of automatically searching for foreign matter on a bare wafer by utilizing compositional contrast given by backscattered electrons.
Since backscattered electron signals well reflect compositional information about specimens as described previously, backscattered electron signals are widely used in surface imaging and analysis instruments including SEM and EPMA, together with secondary electron signals. For example, in Japanese Patent Laid-Open No. 2000-36276, there is disclosed a technique of dispensing with readjustment of contrast and brightness when imaging is done by a computer-controlled electron microscope and when the imaging is done while switching the mode of operation between secondary electron imaging mode and backscattered electron imaging mode.
Where imaging and analysis are performed by SEM or EPMA, it is customary to greatly vary the illumination conditions including accelerating voltage and emission current depending on the state of the specimen and on the purpose of imaging or analysis. When a portion having a different composition, such as inclusions present on the specimen, should be discerned, the brightness and contrast of the backscattered electron image are appropriately adjusted to display an image that is easy to view.
FIGS. 11A and 11B conceptually illustrate the relationships among the brightness of a backscattered electron image, contrast, and the intensity of a backscattered electron signal. Shown in FIG. 11A is a backscattered electron image of a field of view under low-contrast conditions. Shown in FIG. 11B is a backscattered electron image of the same field of view under high-contrast conditions. A specimen has three phases α, β, and M which have their respective average atomic numbers determined by their compositions. An electron beam was scanned over a straight line L. Variations in the intensity of the backscattered electron signal produced at this time are shown in the graphs in FIGS. 11C and 11D. Contrast of a backscattered electron image varies if the illumination conditions including accelerating voltage and emission current are varied or the amplification factor of the amplifier is varied. Furthermore, if the sensitivity of the backscattered electron detector is varied due to contamination, the contrast is varied. If the amplification factor of the amplifier is changed, the DC component of the signal is also varied. Therefore, the brightness is normally adjusted by varying the offset of the amplifier.
However, where specimens of similar kinds, such as different portions of the same specimen, are imaged under various illumination conditions, the user may want to display a backscattered electron image such that portions of the same composition are displayed at the same brightness and contrast. In this case, there is the problem that it is laborious to calibrate the sensitivity of the backscattered electron detector using a reference specimen.
Where a different instrument is used, the sensitivity of the backscattered electron detector equipped to the instrument and the amplification factor of the amplifier are different from those of the previous instrument. Therefore, when the user attempts to compare backscattered electron images of the same specimen, there is no reference against which the measurement can be compared. Consequently, there is the problem that accurate comparisons cannot be made.
In the case of the technique disclosed in Japanese Patent Laid-Open No. S52-117192 or the technique disclosed in Japanese Patent Laid-Open No. H8-201317, it is necessary to strictly maintain constant the relationship (calibration curve) between the intensity of backscattered electrons and the average atomic number of the specimen. Therefore, there is the problem that the same illumination conditions including the accele rating voltage of the electron beam directed at the specimen and emission current must be used at all times or, if the conditions are varied, the calibration curve must be measured again.